The present invention relates generally to apparatus for providing a measure of the distance between the apparatus and an object. In particular, the present invention is directed to automatic focusing systems in which a primary optical means, such as the taking lens of a camera, is moved to maintain an image of the object in focus at the plane of a photographic film.
One highly advantageous type of automatic focus apparatus is the spatial image correlation type. Examples of the different forms of arrangements of this type can be found in copending U.S. patent application Ser. No. 700,963, filed June 29, 1976, by Norman L. Stauffer, and assigned to the same assignee as the present invention; in U.S. Pat. Nos. 3,836,772; 3,838,275; 3,958,117; and 4,002,899; by Norman L. Stauffer, and in U.S. Pat. No. 3,274,914 by K. Biedermann et al.
The typical spatial image correlation apparatus includes two auxiliary optical elements (for example, lenses or mirrors) and two detector arrays. The object distance is determined by moving one of the auxiliary optical elements relative to one of the radiation responsive detector arrays until they occupy a critical or correlation position. This position is a measure of the existing object to apparatus distance.
The relative movement of the auxiliary optical element and the detector array occurs for each distance measuring or focusing operation. The critical condition occurs when there is best correspondence between the radiation distribution of the two auxiliary or detection images formed on the two detector arrays. This condition of best distribution correspondence results in a unique value or effect in the processed electrical output signals. Typically, the correlation signal will contain a major extremum (either a peak or valley) and one or more minor extrema. The major extremum is indicative of the distance to the object.
In most systems, the relative movement of the auxiliary optical element with respect to the detector arrays is achieved by moving a lens or mirror relative to one of the detector arrays. The particular position of the element when best distribution correspondence occurs provides a determination of the existing object of apparatus distance. The position of the auxiliary optical element at the time of best correspondence is used to control the position of the primary optical element, such as the camera taking lens.
In the previously mentioned copending application, Ser. No. 700,963 by Norman L. Stauffer, the correlation signal includes a major peak which is indicative of the distance to an object. A peak detector is used to determine this major peak. A complete scan of all focus zones is provided to insure that the highest correlation is achieved. The location of the last and, therefore, the highest peak detected, corresponds to the desired focus position.
While the arrangement of the Stauffer application Ser. No. 700,963 is generally acceptable, the particular circuit disclosed by Stauffer allows a peak other than the last peak (the correct focus peak) to determine whether focus correction is needed or not.
In my copending application Ser. No. 743,189 filed Nov. 19 , 1976, and assigned to the assignee of the present invention, I provide a continuous automatic focus system that overcomes the shortcomings of the prior art and provides an operation which occurs only with respect to the position of the major extremum to produce a focus correction signal and to drive the lens in the appropriate direction to achieve proper focus. While the system disclosed in the application Ser. No. 743,189 is generally satisfactory, some oscillating or hunting may result from the fact that the signal for the drive motor is the same for large focus errors as it is for small focus errors. This can produce some focus oscillation since with large drive signals near the focus position, overshoot is likely to occur. Reducing the speed of the motor drive would reduce such oscillation but would produce a system that might require too much time to achieve a proper focus condition.